The present invention relates to synchronization systems for clock signal generators utilized in rf receivers.
A typical direct sequence, spread spectrum system is characterized by a transmitter having an output signal comprising a continuous wave carrier phase modulated by a pseudo-random code. The pseudo-random code typically modulates the carrier at megahertz rates, thereby causing the modulated output signal to be spread out over a bandwidth of two times the code modulation rate. Reconstruction of the carrier signal by a receiver requires that a receiver local oscillator be phase-modulated by a pseudo-random code generator having the same pseudo-random code as that of the transmitted pseudo-random code, and that the two codes be synchronized in frequency and have a predetermined phase relationship with respect to each other. Conventional spread-spectrum receivers generally require that each time the incoming signal fades below the receiver threshold, the pseudo-random code generator in the receiver loses lock because of frequency instabilities associated with a clock generator clocking the pseudo-random code generator. Once synchronization is lost, the code generator must be resynchronized with the incoming signal, thereby resulting in both lost time and lost data. One solution to this problem is to clock the receiver pseudo-random code generator by an extremely high stability VCXO that will not drift significantly during periods when incoming signal is lost. However, attempts to use this approach have been generally unsatisfactory because a high stability VCXO is not adapted to track rapid frequency changes in the incoming pseudo-random code caused by variations in propagation path length or other natural phenomena. In addition, the clock signal must be capable of being moved off frequency by several thousand code bits per unit time relative to the transmitted pseudo-random code so that the codes can scan by each other for initial acquisition. All of these constraints upon the clock signal cannot be met by a VCXO having particularly good free-running frequency stability. Thus, conventional spread-spectrum receivers require a reacquisition of the pseudo-random code whenever the incoming signal drops below a predetermined level and synchronization is lost.